This invention relates to apparatus for nondestructive evaluation of materials, and in particular to an x-ray collimating device useful in microfocus projection radiography.
Microfocus projection radiography is an emerging technology used to nondestructively evaluate materials and components. A unique feature of the microfocus system is its small x-ray focal spot, nominally 10 .mu.m in diameter, which allows high magnification projection imaging without significant loss of geometric sharpness. Magnification is essential for detection of small defects, e.g., those less than 100 .mu.m in size.
Conventional radiography, which in general does not allow magnification without loss of geometric sharpness, can introduce imaging anomalies or artifacts. In conventional radiography, these anomalies or artifacts are well understood. However, recent proliferation of microfocus x-ray systems has revealed imaging artifacts peculiar to microfocus projection radiography.
One type of imaging artifact that has been observed with microfocus radiography has become known as the "shadow problem". This term refers to an unexpected increase in radiographic density (excess film darkening) on the x-ray film image of an object. This anomaly occurs with magnifications significantly greater than 1.times., i.e., where the object exposed to radiation is sufficiently distant from the film surface to result in magnification, for reasons described in more detail below. The extent of the shadow increases as the object is moved laterally farther from the centerline connecting the x-ray source and the center of the image plane. The shadow is particularly apparent when film imaging is performed with low accelerating potentials (kilovoltages) where x-ray attenuation is high. The shadow follows the edge contour of the object being radiographed; for example, the shadow on a bar with a straight edge appears straight while the shadow on a bar with a contoured edge traces the contour. Depending on the image geometry, a second shadow can sometimes be observed that is much less significant.
An illustration of two contiguous bars radiographed side-by-side but off the centerline of the x-ray source and image plane is shown in FIG. 1. In FIG. 1, source focal spot 1 emits x-rays 2, some of which are directed toward object 3 comprising bars 4 and 5 disposed between focal spot 1 and film 6. Magnified image 3a of object 3 is produced on film 6 showing images 4a and 5a of bars 4 and 5 respectively. The degree of magnification of images 4a and 5a depends on the relative distance between film 6 and object 3 and between object 3 and focal spot 1. First shadow 7 (darker) and an apparent second shadow 8 (less dark) also appear on the film as a darkening of a portion of the image. Apparent second shadow 8 overlaps an observable line between images 4a and 5a indicating the interface between bars 4 and 5. First shadow 7 typically includes an overlapping portion from an actual second shadow of which the non-overlapping portion appears as shadow 8. The overlapping portion of the actual second shadow may or may not coincide with the entirety of first shadow 7. However, an outer edge of the actual second shadow occurring within the borders of first shadow 7 will not normally be apparent due to the typically greater radiographic density of first shadow 7. Since bars 4 and 5 have straight edges, the edges of shadows 7 and 8 also appear as straight. The shadow effect is observable in all dimensions of the image, proportional to the dimensions of the object. However, for the purpose of illustration, only the most prominent shadow, that of the width dimension, is shown in FIG. 1.
Detection of flaws in materials and components by microfocus projection radiography is dependent on a change in image contrast, caused by some minimal change in object thickness, density, or composition, that is detectable on the radiograph. A flaw such as a crack or a void is imaged as a local increase in radiographic density (darkening) on the radiograph caused by an effective decrease in object thickness at the crack or flaw. The shadow anomaly not only obscures local radiographic density changes but also reduces the overall signal-to-noise ratio of the image. The shadow also, for the same reasons, makes it difficult to define the true edge of the object on the image. This characteristic is critical because knowledge of the proximity of a flaw to the edge of the object is a key factor in determining the severity of the flaw. Thus, suppression or elimination of the shadow anomaly is essential for optimizing the nondestructive evaluation process utilizing microfocus projection radiography.
A certain degree of darkening is typically observed at the edge of the magnified image, due to geometric effects dependent on such factors as object shape and degree of magnification. FIG. 2, in which like features to those in FIG. 1 are designated by the same reference numerals, illustrates the definition of the true edge 9 of object 10 comprising two contiguous bars 11 and 12 positioned side to side. Divergent x-rays 2 from source focal spot 1 are directed as primary radiation toward edge 9 of object 10. The rays travel shorter path lengths within the object, as 13 and 14, through the outer edges of the body, but the path lengths, as 15 and 16, through the main body are nearly constant. A small change in thickness, e.g. that caused by a void or a crack, has a large effect on attenuation at the edge of the body, as between path lengths 13 and 14, showing as a darker area in the image, while the attenuation associated with the main body is nearly constant. This expected geometric effect on the image generated by the primary radiation, however, does not explain the shadow anomaly observed in the film image.
In the course of development of the present invention, the major source of the shadow anomaly in microfocus projection radiography has been identified, and a device has been developed for suppressing the shadow anomaly. The device substantially reduces the severity of image artifacts (shadows) observed in projection radiographs, and improves flaw detection sensitivity for nondestructive evaluation of materials and components.